Monitoring systems for monitoring physical quantities or parameters are used in many applications such as manufacturing plants, chemical processing plants, pipe lines and telephone cables. The physical quantities monitored include fluid pressure, flow rate, temperature, humidity, etc. An important application is that of monitoring fluid pressure in telephone cables. This application typically requires the use of transducers at monitoring points which may be located up to one hundred miles or so from the monitoring or central station. In this kind of application, it is desired to use a large number of transducers with a minimum of transmission lines and power requirements while realizing a high degree of accuracy and reliability in the acquisition of data representing the values of the monitored physical quantities.
In the prior art, the Goldstine et al U.S. Pat. No. 3,327,289 discloses a remote monitoring system especially adapted for multiple transducers. The central station is connected through a pair of wires to multiple remote transducers each of which is selectively responsive to a tone frequency. An interrogator circuit at the central station selectively applies the tone frequencies to the pair of wires and a constant current source is continuously connected to the pair of wires. When a transducer receives the tone frequency assigned to it, a switching transistor is turned on and a variable resistor representing the physical quantity is connected across the pair of wires. Compensation is provided for the resistance of the wires and the voltage appearing on a resistor at the central station is proportional to the value of the quantity being monitored.
The Whiteside U.S. Pat. No. 4,103,337 discloses a monitoring system in which data is transmitted from a remote sensor using a form of analog to pulse width to digital conversion. A central processor transmits a data request signal on a single wire transmission line to the remote sensor. The data request signal activates the sensor and the interfaced electronics and also provides electrical power thereto. Upon receipt of the data request signal, the interface electronics generates a low impedance to ground signal on the transmission line after a time lapse corresponding to the value of the quantity being monitored. The low impedance to ground signal is detected by the central processor which terminates the data request signal. The central processor includes a pulse width to digital converter which produces a digital signal indicative of the time duration of the data request signal and hence the value of the quantity.
The Gowen U.S. Pat. No. 3,438,019 relates to a monitoring system especially adapted for measuring voltages at selected test points in complex electronic systems. This patent discloses a central computer with multiple remote stations for monitoring a voltage at each station. Each station senses the test voltage and a voltage-to-frequency converter produces a pulse train having a frequency which is a function of the sensed voltage. The individual sensors at different locations are gated in sequence. When the output of a voltage-to-frequency converter is gated on, the data signal is transmitted to the central computer where the frequency is measured and compared with standard values corresponding to the particular test point. The standard values are stored in a computer memory and accessed according to the test point being interrogated. The system does not give a value readout of the voltage being measured; instead it merely tells whether the measured value is within limits. The system includes a voltage source at each remote point for energizing the voltage-to-frequency converter.
The Connelly U.S. Pat. No. 3,629,855 relates to a transducer monitoring system for use in connection with process control systems. In the system of this patent, multiple transducers are remotely located and each is connected with an intermediate level selector by individual transmission lines. There are plural intermediate level selectors which are connected to a master level selector which, in turn, is connected with an address scanner and a computer. When a signal condition changes at a transducer, the associated receiver generates an event flag. This causes a signal to be sent through the intermediate level selector to the master level selector and the address scanner searches the intermediate level detectors and the individual transducers to identify the one which transmitted the event flag. When the transducer is found, its address and the sense of the signal change are recorded by the central computer. The signal conditioner is described as a particular logic level pulse-width encoder and additionally the patent mentions other signal conditioners, such as voltage-to-frequency converters.
Other patents which disclose remote monitoring systems are Swanson, No. 4,090,248; Shih, No. 4,123,796; Actor et al, No. 4,153,945 and Iechenlaub, No. 4,156,929.
A general object of this invention is to provide an improved transducer for use in a monitoring system.